As discussed earlier, Goldman's entire S&P500 price forecast for 2018 and the next three years is based on two things: tax reform passing, but more broadly, something that David Kostin dubbed "Rational Exuberance", to wit:

"Rational exuberance" best describes our forecast for the trajectory of the S&P 500 during the next several years. Earnings drive stocks over time and should support the index rising to 2850 at year-end 2018, 3000 at the end of 2019, and 3100 by the close of 2020, representing a price gain during the next three years of 20%. Our price targets imply a modest expansion in forward P/E multiple to 18.2x at year-end 2018, a flat multiple in 2019, and a contraction to 18.1x in 2020.

So far so good, but as Kostin also explained, absent tax reform passing, the S&P will not only not hit 3,100 in 3 years, it may well be lower: "Assuming tax reform passes, we forecast S&P 500 adjusted EPS will jump by 14% to $150 in 2018. Equity investors will be rewarded as the index advances by 11% to 2850 at year-end 2018 and delivers a total return of 13% including the 2% dividend yield."